Resins that part of the hydroxyl groups of polyhydroxystyrene is substituted with tert-butoxycarbonyloxy (t-Boc) groups have been disclosed as base polymers to be used for materials of chemically-amplified resists (in Japanese Patents Laid-open Nos. Sho 59-45439 and Sho 62-115440 and others).
A variety of poly(p-hydroxystyrene)s partially protected with the tert-butoxy group, that is, the hydroxyl groups of poly(hydroxystyrene) are partially substituted with tert-butoxy groups, and processes for their preparation have also been proposed.
Firstly, the poly(hydroxystyrene) partially protected with the tert-butoxy group is produced by copolymerization of tert-butoxystyrene and p-hydroxystyrene. A disadvantage of this process is however that p-hydroxystyrene is a highly self-polymerizing monomer with difficulties of handling and polymerization control.
Secondly, poly(p-hydroxystyrene) is reacted with isobutene to give the poly(hydroxystyrene) partially protected with the tert-butoxy group. In this process, the synthetic route is very complicated such that a hydroxystyrene derivative with its hydroxyl group protected is first polymerized, then the hydroxyl group of the obtained polymer is deprotected and further the product is reacted with isobutene. Besides, isobutene cannot be added to the hydroxyl group as calculated from the amount added. Therefore, it is difficult to introduce a designed quantity of the tert-butoxy group.
Thirdly, the poly(hydroxystyrene) partially protected with the tert-butoxy group is produced by a partial elimination reaction of poly(p-tert-butoxystyrene). In this process, poly(p-tert-butoxystyrene) is synthesized by radical or anion polymerization or the like of p-tert-butoxystyrene, and the tert-butoxy group of the polymer is partially removed with an acid catalyst.
In this process, simple changes of addition amounts of an acid catalyst required for the reaction did not result in the production of the poly(hydroxystyrene) partially protected with the tert-butoxy group as designed, because the elimination reaction proceeds catalytically. A tert-butoxy group ratio [a ratio of the tert-butoxy group bound to the phenolic hydroxyl group in poly(hydroxystyrene) partially substituted with the tert-butoxy group] is particularly one of important factors to determine resist characteristics such as a dissolving rate in a developer. It is therefore very important to accurately control the tert-butoxy group ratio for producing highly sensitive resists with high resolution. It is therefore necessary to develop processes for producing poly(hydroxystyrene)s partially substituted with tert-butoxy groups with the tert-butoxy group ratios as designed.
To solve the above problems, Japanese Patent Laid-open No. Hei 8-1047808 has disclosed processes for the preparation of poly(hydroxystyrene)s partially protected with the tert-butoxy groups by that poly(p-tert-butoxystyrene) is reacted with an acid catalyst at a molar ratio of 0.05 to 2.0 to the tert-butoxy group of the polymer, changes in the solubility in water of the produced poly(hydroxystyrene) partially protected with the tert-butoxy group are determined, and the reaction is terminated when the desired amount of the tert-butoxy group is eliminated. This process is simpler than the above first to third processes, and makes it possible to control the degree of deprotection.
A drawback of this process is however requirements of quantitative analyses of the solubility of the product, poly(hydroxystyrene) partially protected with the tert-butoxy group, in water by sampling the reaction solution at fixed intervals. A quantitative analysis needs a certain time to complete. There was a limit to the control of the degree of deprotection because the reaction proceeded while the quantitative analysis was carried out. More accurate control of the tert-butoxy group ratio is particularly required to meet today's demand for more highly sensitive resists with higher resolutions.
The solubility of a polymer is by nature in a certain range. It is difficult to control the degree of deprotection, for example, within ±2% of the target degree, only by observing solubility changes.